One researcher says so but others aren't convinced

Action Points

After a 35-year downward trend in U.S. road deaths per population, 2015 saw a 7% spike and one researcher believes climate change may be the cause.

Note that the researcher suggested several preventive measures that could reduce road deaths and severe injuries, such as lowering maximum speed limits on urban freeways and installing speed bumps in residential areas.

After a 35-year downward trend in U.S. road deaths per population, 2015 saw a 7% spike and one researcher believes climate change may be the cause.

Writing in the journal Injury Prevention, Leon Robertson, MD, an injury epidemiology and prevention expert now retired from Yale University School of Public Health, reported that the increase in fatalities from 2014 to 2015 is likely due to an increase in road activities encouraged by warmer temperatures.

According to his analysis of roads deaths per billion person-days and average temperature on days of deaths, the higher the temperature, the higher the death rate (r2=0.51, P<0.001): "As temperatures continue to increase from heat-trapping gases in the atmosphere, road deaths will likely increase more than expected unless there are major mitigating countermeasures," he explained.

Although safety officials attributed the increase in road deaths to an increase in cell phone use, Robertson asserted that cell phone use increased in the past without reversing the road death trend, suggesting that there must be another explanation.

"Use of cell phones or other distractions while driving increases risk, but did so gradually as the devices became popular," he told MedPage Today. "It is common knowledge that many people consult weather forecasts before engaging in outdoor activity. I knew that 2015 had record high temperatures in many areas of the country, so I decided to test the hypothesis as best I could without data on how much people walk or bicycle in relation to temperature."

Asked for his opinion of the study, Doug Brugge, PhD, of Tufts University in Boston, noted that the analysis showed an interesting association but that "there were some assumptions and limitations."

"My question is what about seasons, day length, and sun levels? There needs to be more research in the area," he said in a telephone interview.

Similarly, George D. Thurston, MD, director of the Program in Human Exposures and Health Effects at NYU School of Medicine, agreed that the temperature-accident causal mechanism was not well enough justified: "Smartphone use -- [which makes texting easier] -- has kept increasing, and has even accelerated after 2014, so that seems a plausible explanation. [Robertson] should have addressed that factor directly in the analysis, rather than dismissing cell phone effects up front."

For the study, Robertson analyzed two sets of data -- one to assess the link between temperature and rainfall on annual miles driven per person in urban areas of the U.S. and a second to examine the relationship of weather and death risk per population at the county level.

He estimated the effect of temperature and rainfall on miles driven during 2010, and then matched date and county data with temperature and the number of people exposed to that temperature in order to calculate the risk per persons exposed to specific temperatures. Using logistic regression analysis of temperature and precipitation in 2014, he projected the expected deaths in 2015 among the 100 most populous counties in the U.S.

He found that vehicles were driven an average of 60 more miles per year per person for each increase in a degree of temperature (95% CI 8.42-110.62). Additionally, people drove their vehicles an average of 66 more miles per year for each one inch increase in precipitation (95% CI 36.62-97.96).

"The finding that vehicles are driven an average 60 more miles per person for each degree increase in temperature may not seem like much, but applied to the sum of the total populations in the areas included, the increase is substantial," he stated.

The death rate was higher in warmer areas and in those with higher rainfall, as well as in counties with larger land mass and higher speed limits on urban freeways. Conversely, the death rate was lower in households with higher average incomes, possibly because drivers were able to afford newer models with better safety features, he speculated.

He mentioned several preventive measures that could reduce road deaths and severe injuries, such as lowering maximum speed limits on urban freeways and installing speed bumps in residential areas.

"Walking and bicycling are healthful exercises but are best pursued on off-road trails, parks, athletic fields, malls, etc. Some cities provide biking and walking venues separate from traffic. Where such are not feasible, barriers between motor vehicles and pedestrians or bicyclists are an option."

Robertson cautioned that a major limitation of the study is that it was observational, so no firm conclusions can be drawn about cause and effect.

Robertson reported no financial disclosures of interest.

Reviewed by Robert Jasmer, MD Associate Clinical Professor of Medicine, University of California, San Francisco and Dorothy Caputo, MA, BSN, RN, Nurse Planner

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